Set custom location for import/export helper
My guess is you want the filebrowser to open at this location. My suggestion on this would be to set up default path properties in your addon preferences (if an addon).

class ImportDefaultPathsAddon(AddonPreferences):
    # this must match the addon name, use '__package__'
    # when defining this in a submodule of a python package.
    bl_idname = __name__

    default_obj_path = StringProperty(subtype='DIR_PATH')
    default_collada_path = StringProperty(subtype='DIR_PATH')

    def draw(self, context):
        layout = self.layout
        layout.label(text="Import Paths")
        layout.prop(self, "default_obj_path")
        layout.prop(self, "default_collada_path")


Then in your layouts, menu  or panel button

def draw(self, context):
    user_preferences = context.user_preferences
    addon_prefs = user_preferences.addons[__name__].preferences

    layout = self.layout
    op = layout.operator("import_scene.obj")
    op.filepath = addon_prefs.default_obj_path

    op = layout.operator("import_scene.collada")
    op.filepath = addon_prefs.default_collada_path


The user would need to set the paths when the addon is enabled.

PS I've looked at other methods of doing this, and can also suggest ditching the ImportHelper subclass altogether and write your own invoke and parameters.